There exists in the state of the art electrooptical devices exhibiting a phenomenon known as persistent electrochromism wherein electromagnetic radiation absorption characteristics of a presistent electrochromic material are altered under the influence of an electric field. Such devices are employed in sandwich arrangement between two electrodes. Coloration is induced by charging the electrochromic film negative with respect to the counter electrode which can be the same as the persistent electrochromic material or different. By reversing the original polarity of the field or by applying a new field it is possible to cancel, erase or bleach the visible coloration. These steps of color induction and erasure are defined as cycling. As described in U.S. Pat. No. 3,978,007, presistent electrochromic material is defined as a material responsive to the application of an electric field or a given polarity to change from a first persistent state in which it is essentially non-absorptive of electromagnetic radiaiton in a given wave length region to a second presistent state in which it is absorptive of electromagnetic radiation in the given wave length region and once in said second state is responsive to the application of an electric field of the opposite polarity to return to its first state. Certain of such materials can also be responsive to a short circuiting condition, in the absence of an electric field, so as to return to the initial state. Further, by "persistent" is meant the ability of the material to remain in the absorptive state to which it is changed after removal of the electric field as distinguished from a substantially instantaneous reversion to the initial state.
Thus, in this case, the device functions by applying an EMF to the electrodes to cause coloration or a visible display of the electrochromic layer receiving the applied EMF. By reversing the polarity of the two electrodes and applying an EMF, the colored electrochromic layer will be uncolored (erased or made invisible) by the reverse flow of current. The film will be persistent in either its colored (visible) state or its noncolored (invisible) state without the need for a continuous current or EMF to maintain the state. See also U.S. Pat. No. 4,344,674.
As stated in U.S. Pat. No. 4,174,152 the exact mechanism of persistent electrochromism is unknown but the coloration is observed to occur at the negatively charged electrochromic layer. Generally, the phenomenon of persistent electrochromism is believed to involve transport of cations, such as hydrogen or lithium ions, to the negative electrode where color centers form in the electrochromic image layer as a result of charged compensating electron flow. As stated in the prior art patents, voltages of less than two volts d.c. applied to the layers of the electrochromic material will bring about the color change and reversal Thus, the devices require very little current or power to operate and yet can be effectively colored (made visible) and discolored (made invisible) by the reversal of the small voltage potential to the material.
It is also known that liquid crystals work in a similar manner wherein liquid crystals used in displays are not optically active but instead change their light scattering characteristics when a voltage is applied across a liquid crystal film between two closely spaced conductive sheets. With no applied voltage, the crystalline structure is orderly and the material is clear and invisible; with voltage applied, the light is scattered and the layer becomes opaque and visible. Liquid crystals have been classified into three categories: nematic, cholesteric and smectic. The nematic and cholesteric phases are liquid which have optical properties. The optical absorption spectrum of a pleochroic dye molecule is a function of its molecular orientation with respect to the polarization of the incident light. If the dye molecule is oriented with its long axis parallel to the electric vector of incident polarized light, absorption of the light by the molecule occurs, and an observer sees the characteristic color of the dye. Conversely, if the dye molecules have their long axis perpendicular to the electric vector, little or no absorption occurs and the incident light is transmitted unchanged.
Nematic compounds have a strong dipole moment on the molecule's main axis and thus orient dye molecules in color switching devices. Such electronic color switching relies on the introduction of dyes composed of pleochroic molecules. To orient the dye with respect to the field to produce the desired color changes requires the use of a nematic liquid crystal that has different properties than the type used for dynamic scattering. The nematic material to which the dye is added must have an extremely strong dipole moment operating along the long axis of the molecules instead of one operating at an angle with this axis. When a voltage is applied, this moment will align with the field. This causes the dye molecules to also align with the field resulting in the disappearance of the color of the dye. Removal of the voltage collapses the field; the nematic host molecules with their dye guest realign in their original configuration and the cell returns to its original color. See Joseph A. Castellano, "Now That The Heat Is Off, Liquid Crystals Can Show Their Colors Everywhere", Electronics, pages 64-69, July 6, 1970. See also "The Bulletin of Arthur D. Little, Inc., No. 488, March-April, 1971.
Further, it is well known that one of the most striking transformations of the optical properties of the cholesteric phase of liquid crystals are those due to temperature changes. Although cholesteric materials are substantially colorless in the isotropic liquid phase, when they are cooled through their clearing temperatures, some pass through a series of colors as viewed in reflected light. Some change only from red to green on cooling; others change from red to green to blue or from red to green and back to red; some are initially blue and change to green and then red. The most remarkable property is that each color corresponds to an exact temperature of the cholesteric material. The rate of color change, the temperature at which specific colors occur and the direction of shift are predictable. See James L. Ferguson et al, "Liquid Crystals And Their Applications" Electro-Technology, page 41, January, 1970.